These universal joints are used for such operating gears as water sliding doors, sky lights, valves for ventilation, valves for piping, main engine revolution indicators, or transmission gears in ships. They are classified into 4 main types according to the shape of the coupling applicable to the transmission shaft and fixing method of the shaft as shown below. And also classified into four types, AA, BB, BB1 and AC by combining the above types. See illustrations above. Dimensions of steel pipes for transmission shaft for Type A. Materials are usually steel or, stainless steel. Specify when ordering.